It is known that single-phase induction motors can be used in different applications, such as in household appliances in general, refrigerators, compressors among others, by virtue of their simplicity, robustness and performance.
Induction motors are comprised of a squirrel cage-type rotor and a stator, the latter being provided with a main winding and a start winding. During its operation, the main winding is energized by a source of alternating voltage, while the start winding is energized temporarily, thus creating a rotating magnetic field in the stator air gap, a condition that is necessary to accelerate the rotor and promote its starting.
In general, the rotating magnetic field is obtained by energizing the start winding with a time-lagged current with respect to the current in the main winding. Phase shift is preferably at an angle of 90 degrees between the currents of the windings.
As it is general knowledge to one skilled in the art, phase shift may be obtained by means of constructive characteristics of the windings or by means of an external impedance in series with the start winding. Since the value of the current circulating in the start winding is relatively high, it is observed that it is necessary to interrupt this current after a certain period of time.
Such interruption is made by means of a starting device, such as a PTC, electromechanical relay or temporizer-type starting components. Examples of these devices are cited in patent documents U.S. Pat. No. 5,053,908 and U.S. Pat. No. 5,051,681, and in the publication of the PCT international patent application WO 02/09264 A1.
In relation to the PTC, it is observed that the problem with this device relates to the optimization of the time of energizing the start winding. Its conduction time is directly proportional to the volume of the ceramic tablet and inversely proportional to the circulating current, which entails a reduced starting time when it is applied to motors having higher power and a too long time when it is applied to smaller motors. These two factors lead to starting deficiency of larger motors and a higher consumption of energy during the starting period in smaller motors.
With regard to the electromechanical relay, it is noted that this one has as main deficiency the need of dimensioning of a specific component for each size of electric motor, preventing its use in high efficiency motors.
Referring now to the temporizer-type starting components, it can be seen that these do not adapt the starting time required for different motor sizes. Their concept does not enable one to size a circuit that could provide optimized starting time for the different motor sizes, and so the existence of various models becomes necessary, each of them adjusted for a determined actuation time, so as to meet a determined family of electric motors, which will cause lack of standardization, high adjustment in the manufacturing lines and increase in stocks.
In order to correct the problems of these known devices, some solutions have been developed. For example, U.S. Pat. No. 6,320,348 discloses a starting circuit for a single-phase motor. Such a circuit is based on the measurement of electrical quantities (voltage or current) of the motor and a signal of current or voltage variation as a function of time (derivative—di/dt or dV/dt). If the signal di/dt or dV/dt is lower (or higher) at a given parameter, the start winding is disconnected.
Another solution is disclosed in U.S. Pat. No. 8,508,374 which relates to a starting circuit for a single-phase motor. In one of the proposed embodiments, it is mentioned that the control operates from monitoring the zero-crossing of the voltage/current waveforms which feeds the motor windings.
In addition, U.S. Pat. No. 4,361,792 discloses a control system based on the phase shift between current and voltage of an induction motor. This document is based on a first parameter obtained from the actual phase shift between voltage and current, a second parameter obtained from the zero-crossing of the current waveform and a third parameter representing the phase shift desired for voltage and current.
An additional solution is disclosed in U.S. Pat. No. 6,034,503 relating to a method for reconnecting the start winding of a motor. It is mentioned that the start winding is disconnected as soon as the electric current of the main winding reaches a certain value and will only be reconnected if the voltage on the start winding establishes a certain relationship with the electric current on the main winding.
Similarly, the solution disclosed in U.S. Pat. No. 9,160,259 describes a starter device configured to energize the start winding in the condition when a first voltage signal sampled from the voltage source, through a sensor, is lower than or equal to a first voltage level or is higher than or equal to a second voltage level, so as to initiate a starting period of the motor.
Another similar solution is mentioned in U.S. Pat. No. 7,471,058 which relates to a starting device in which a control unit receives from a current sensor signals relating to the current level supplied to the stator, the control unit being connected to the windings to open and close a starting switch when a ratio between the present current level supplied to the stator and the current starting level reaches a predetermined value.
Finally, the solution disclosed in U.S. Pat. No. 9,184,678 relates to a starting device for induction motors, wherein a processing device receives current signals from a current sensor and voltage signals from a current zero-crossing sensor, the processor activating and deactivating the switch according to the signals and with time sensors.